34 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



lower molecular concentration than does sea water, and are independ- 

 ent of it in this respect. 



Fresh-water animals. — The primarily aquatic fresh-water animals 

 are without doubt derived from marine ancestors and must be sup- 

 posed to have inherited from them a similar molecular concentration 

 of their body fluids. As a matter of fact, among such fresh-water 

 invertebrates as the pond clam, Anodonta, and the crayfish, Pota- 

 mobius, the body fluids have a higher osmotic pressure than that of 

 the water in which they live, although not as high as that of sea 

 water. In Anodonta these fluids contain ten times as much dissolved 

 material as the surrounding water. A continuous stream of water must 

 therefore diffuse through the semipermeable body membranes, dilut- 

 ing the body fluids, swelling the body, and interfering with the normal 

 functioning of the protoplasm. To enable fresh-water animals to exist, 

 there must either be devices which prevent the entrance of water, by 

 changes in the permeability of the membranes, or the excretory organs 

 must be capable of excreting the water as fast as it enters. 



Investigation has shown that this second method is certainly em- 

 ployed in many cases. All fresh-water Protozoa have one or more 

 contractile vacuoles which constantly discharge water from the body. 

 At 20°C. Paramecium excretes almost five times its own volume of 

 water in an hour. Marine and parasitic protozoans which live in a 

 medium isotonic with their body fluid have no contractile vacuoles. 

 Complete proof that the contractile vacuole counteracts the osmotic 

 influx of water is furnished by experiment. If a fresh-water amoeba is 

 introduced into sea water by gradual increases in salinity, it will con- 

 tinue to live, but its contractile vacuole ceases to function and ulti- 

 mately disappears. 4 Increase in the molecular concentration of the 

 surrounding water slows down the contraction rhythm of the vacuole 

 and lessens the amount of water pumped out. At about 20°C. the 

 following relations exist: 5 



%NaClinwater 0.25 0.5 0.75 1.00 



Contraction period in seconds 6.2 9.3 18.4 24.8 163.0 



Excretion per hour in body volumes 4.8 2 . 82 1 . 38 1 . 08 0.16 



In Metazoa the kidneys play the same role in removing water taken 

 in by osmosis. In cercaria parasitic in Limnaea — i.e., from snail blood, 

 an isotonic medium— it is usually difficult to recognize the bladder on 

 account of its small extent. When the animal is immersed in tap water, 

 the Y-shaped bladder quickly becomes evident on account of its dis- 

 tention. This shows that the water diffused into the body is being re- 

 moved by the nephridial system. Isopods, amphipods, and decapods of 



